1. Field of the Invention
The present invention relates to an ion beam irradiating apparatus including an ion beam neutralizer.
2. Description of the Related Art
A typical ion beam irradiating apparatus employing an ion beam neutralizer to neutralize the charge of an atomic or molecular ion beam, will now be explained with reference to FIG. 1.
An ion beam irradiating apparatus, shown in FIG. 1, includes an ion source 1 an ion beam drawing electrode 2, a decelerating electrode 3, a mass separator 4, an aperture 5, an ion beam neutralizer 6, and a sample 7. The ion source 1 is usually of a type utilizing the impact of thermoelectrons against a gas, and an ion beam generated from this source is drawn out by the ion beam drawing electrode 2. Then, the ion beam passes through the ion beam decelerating electrode 3 and at this electrode ion energy is controlled.
The ions generated from the ion source 1 consist of those of univalent to tetravalent or pentavalent, and a specified type of ion is selected during passage through the mass separator 4. The mass-separated ion beam 9 finally collides against the sample 7 and reforms it. The aperture 5 controls the quantity of the ion beam 9.
Because an ion beam consists of charged particles and secondary electrons are emitted when the beam impacts against a sample, if the degree of electrical insulation of the sample is high, the potential of the, sample is increased and a dielectric breakdown may occur. In the ion beam irradiating apparatus, a neutralizer 6 controlling the potential of the sample is employed to prevent the dielectric breakdown.
FIG. 2 is a block diagram of a conventional ion beam neutralizer 6.
In FIG. 2, the neutralizer 6 includes a thermoelectron emitting source 11, (e.g. a filament), and a thermoelectron controlling means 12 for controlling the emission quantity of thermoelectrons to be supplied to the ion beam 9 provided between the ion beam trajectory, or orbit, and the filament 11. Filament 11 may be an electron drawing electrode, e.g. an electrode of grating structure enabling thermoelectrons to pass through it. An electron shower 13 is indicated by dotted lines. An electric power supply 14 is used for heating, and there is a drawing electrode 15. Following are explanations on the ion neutralization operation of the apparatus.
To facilitate the emission of thermoelectrons, the surface temperature of the filament 11 is raised to 2,000 to 2,500.degree. C. with supply 14. Next, when a positive potential in comparison with that of the filament 11 is applied to the drawing electrode 12, thermoelectron emission begins and the electron shower is formed. These thermoelectrons collide with the ion beam 9 or flow into the sample 7, so that the sample 7 is prevented from a potential increase.
In the Japanese Patent KOKAI (Disclosure) No. 59-204230, another neutralizer as shown in FIG. 3 is disclosed.
This ion beam neutralizer includes a drift tube 21, a filament 22 arranged in the drift tube 21, an alternating current (AC) power supply 23 to heat the filament 22, a power supply 24 to apply an accelerating voltage to the drift tube 21, a power supply 25 to apply a decelerating voltage to the drift tube 21, and an ion beam 26. As shown in FIG. 3, the ion beam 26 flowing into the drift tube 21 is accelerated or decelerated by the electric field of the drift tube 21. Next, this ion beam 26 is neutralized during passage through the filament 22 which emits thermoelectrons heated by the alternating current power supply 23, and flows out of the drift tube 21 as a neutral beam.
The conventional neutralizing apparatus constructed as described above have the following various problems. The filament 22 is usually made of a thin rod of 0.5 to 1.0 mm in a diameter wound in spiral, and is supported at only two places in the filament leading-out portion. Therefore, when the filament 22 is heated up to about 3,000.degree. C., it thermally deforms, resulting in undesirable enlargement in diameter or unevenness in winding pitches causing difficulties in controlling the degree of neutralizing effect. The distance measured from the filament 22 to the ion beam 26 is constant for an ion beam with a circular cross-section, so that uniform neutralization is effectively performed, whereas for an ion beam with a semicircular or a triangular cross-section, the distance between the filament 22 and the beam 26 unevenly varies and, therefore, uniform neutralization cannot be obtained. It also makes the control of neutralization difficult, so that this is another problem in a conventional neutralizer.
Other conventional ion beam irradiating apparatus as shown in FIG. 4 and FIG. 5 have been proposed.
One ion beam irradiating apparatus as shown in FIG. 4 includes an ion source 31, ion drawing electrode 32, a decelerating electrode 33, a mass separator 34, a deflector 35, an ion beam neutralizer 36, a sample 37, and a rotating table 38 rotatable in the direction of an arrow "A" for holding a sample such as a large diameter wafer. Reference numeral 41 is a thermoelectron emitter, for example, a filament made of a tungsten wire. Reference numerals 42 and 43 are electrodes for forming a thermoelectron control means and are arranged on concentric circles centering an ion beam trajectory. The electrode 42 is arranged between the ion beam trajectory and the filament 41 and controls the emission quantity of thermoelectrons to be supplied to the ion beam. The electrode 43 is arranged between the ion beam trajectory and the electrode 42 so as to control the energy of thermoelectrons. These electrodes are formed into, for example, grating structures enabling electrons to pass therethrough. Reference numeral 44 shows the emitted electron shower. As an ion source 31, a type utilizing the impact of thermoelectrons against a gas is generally used and the ions generated from this source are drawn out by the ion drawing electrodes 42. They then pass through the decelerating electrode 43. The ions generated from the ion source 31 consist of those of univalent to tetravalent or pentavalent and a kind of ions is selected out of them when theY pass the mass separator 34 and other ions are rejected from the ion beam trajectory. Further, the ion beam deflected in the direction of an arrow "B" by the deflector 35 is neutralized by passing through the neutralizer 36 to collide against the specimen 37 held on the rotating table 38. The sample 37 is held on the rotating table 38 rotating at a high speed, so that when the beam is deflected to one direction by the deflector 35 the beam can irradiate the whole surface of the sample 37. The colliding ion beam contributes to surface reformation or film formation. FIG. 5 shows a longitudinal cross-sectional view taken along the line 4--4 of the neutralizer 36.
Now, by reasons that an ion beam consists of charged particles and secondary electrons are emitted when they collide against the sample 37, if the degree of insulation of the sample 37 is high, the potential of the sample is raised and dielectric breakdown may be induced. In this conventional ion-beam irradiating apparatus, the neutralizer 36 for controlling the potential of the sample 37 is employed to prevent dielectric breakdown.
The explanations for the ion neutralization operation of the neutralizer 36 will now be made in the following.
At first, the surface temperature of the filament 41 is increased up to 2,000 to 2,500.degree. C. to facilitate emission of thermoelectrons. Next, when a positive potential in comparison with that of the filament 41 is applied to the electrodes 42 and 43, the emission of thermoelectrons is started and the electron shower 44 is formed. The difference between electrodes 42 and 43 is that the electrode 42 located close to the filament 41 can change the electric field intensity adjacent to the filament 41, to control the quantity of the electron shower. On the other hand, the electrode 43 located close to the ion beam can change the electric potential distribution in the ion-beam passing region as the ion beam passes through the neutralizer 36, to control the energy intensity of the electron shower.
The electron shower 44 collides with the ion beam or flows into the sample 37, so that the potential rise of the sample 37 can be avoided.
In the case of a conventional ion beam irradiation apparatus as shown in FIG. 5, electrodes 42 and 43 are positioned on concentric circles centering the ion beam. If the inner diameters of electrodes 42 and 43 are made large enough, the control of the potential distribution along the axis becomes difficult, so that the inner diameters cannot be freely enlarged. Therefore, the inner diameter is generally designed to be several times as large as that of the ion beam. Because of this fact, if the ion beam is deflected in the direction of the arrow B more than a normal value even in a small quantity, then the beam impinges on the electrode 43 giving damage and spattering to it. Generally, the electrode, 43 is formed by heavy metal such as tungsten or tantalum, and there is another problem that the sample is polluted by the heavy metal.
To prevent such a spattering, a smaller deflection angle by deflector 35 is preferable, but if the deflection angle is made smaller, the distance between the sample 37 and the neutralizer 36 needs to be larger to make the deflection distance uniform. It results in making the ion beam irradiating apparatus itself large.
As described above, the conventional ion beam irradiating apparatuses have many problems in irradiating samples especially having large diameters.